This invention relates to a method and apparatus preferably used in a facsimile system or an image reader system so as to correct both a white document signal distortion and a black document signal distortion included in an image signal generated from an image reader reading a pattern on a document. More particularly, this invention relates to an image signal correcting method and apparatus suitable for reducing the size and scale of the system of the kind described above.
A conventional apparatus for correcting an image signal is disclosed in, for example, JP-A-61-12175. The disclosed apparatus comprises means for A/D converting an image signal generated from an image sensor, means for storing a dark signal output of the image sensor, means for subtracting the dark signal output from a bright signal output of the image sensor thereby storing a sensitivity component, and means for processing the sensitivity component together with the signal component obtained by subtracting the dark signal output from the image output signal of the image sensor, whereby the image output signal of the sensor having both a dark signal distortion and a bright signal distortion can be corrected to be freed from such distortions.
In the disclosed apparatus, the image signal in the form of the digital signal is processed for the purpose of correction of the image signal. Therefore, the conventional apparatus requires additional processing means besides the means for storing the dark signal output of the image sensor and the means for storing the sensitivity component. That is, a memory having a function required for executing high-speed processing is additionally provided. Thus, a memory having a large memory capacity, for example, a 1-megabit memory is additionally required, and it has been difficult to reduce the scale of the apparatus, to integrate the apparatus into an integrated circuit and to reduce the cost of the apparatus.
Further, when the image sensor is brought into direct contact with a document to read an image of a pattern on the document, part of light from light sources may impinge on the sensor itself thereby causing a shading distortion in the image signal. Such an undesirable phenomenon was not taken into consideration in the apparatus described above.